BEIJING (AP) ― Around 20 attorneys gathered Saturday in a central Chinese city to demand access to several activists detained ahead of the politically sensitive 25th anniversary of the Tiananmen Square military suppression of protesters.

Prominent rights lawyer Li Fangping said the attorneys staged an overnight protest starting Friday outside the Zhengzhou city police bureau after authorities denied their requests for weeks to see their clients.

“We feel that the entire system for defense lawyers is facing an enormous challenge,” said Li, who was among those who protested.

Among the activists detained in Zhengzhou is the well-known lawyer Chang Boyang, who has campaigned for the rights of migrant workers, HIV patients and children poisoned in a 2008 tainted milk scandal.

Chang was detained May 27 as part of a string of prosecutions against artists, lawyers, scholars and journalists ahead of the June 4 Tiananmen anniversary amid intense government efforts to deter public commemorations.

Li said senior city police representatives in Zhengzhou met with the lawyers Saturday afternoon and said the attorneys had to seek permission from police investigators in order to meet their clients.

Under Chinese law, such a requirement is necessary only in cases such as major corruption, terrorism or subversion, Li said, and the activists in Zhengzhou were accused of public disturbance offenses, which did not fall under that category. The officials told the lawyers their justification was that the detainees’ cases were related to state security concerns, Li said.

“This is setting a very bad precedent in which if you were detained even for the most ordinary offense, you won’t be allowed to see you lawyer on the claim that it endangers state security,” Li said. “This is a subversion of Chinese law.”

Reached by phone, officials in the propaganda departments of Zhengzhou’s police and Communist Party offices refused to comment.

Discussions of the 1989 Tiananmen Square protests and its military suppression are taboo in China, and authorities tighten security ahead of the anniversary each year. But this year’s suppression was harsher than in previous years, as police rounded up activists who had received only warnings in the past.